1. Field of the Invention
Present invention relates to polyester compositions, and more particularly to poly(ethylene terephthalate) (PET) thermoplastic molding compositions containing a triglyceride oil nucleating agent.
2. Discussion of the Background
Poly(ethylene terephthalate) is a widely utilized polymer due primarily to its high strength, thermal stability, good barrier properties, its low price, and good solvent resistance. PET has established itself as a primary material in bottles, textiles, recording tape and packaging film. Injection molding is, however, one area in which, until recently, PET has not been extensively utilized, due to its slow crystallization rate which results in poor dimensional stability of molded parts. The crystallization rate of PET on cooling from the melt to the glass is considerably slower than that of poly(butylene terephthalate) (PBT) due greatly to the difficulty in forming chain folds. Its maximum crystallization rate occurs at about 180.degree. C., roughly halfway between its glass transition temperature (approximately 75.degree. C.) and its peak melting temperature of about 250.degree. C.. At 90.degree. C., a typical injection mold temperature for polyesters, the rate of crystallization for PET is virtually zero. Although one method of improving the moldability of PET has been to raise the temperature of the mold, at temperatures much greater than 110.degree. C. it is no longer possible to use water to heat the mold, so that oil must be utilized instead. Since most molders do not wish to switch to oil heating when molding PET, nucleation agents and mobility enhancers have been developed which allow mold temperatures of 100.degree. C. or lower. Thus, much of the prior art which has been developed in the area of PET molding concerns the development of nucleating agents which are effective in inducing rapid PET crystallization at high temperatures when cooling from the melt temperature to the mold temperature.
Developments in the 1970's and early 1980's (see for example U.S. Pat. Nos. 4,429,067; 4,352,904; 4,351,757; GB 1,292,679) in PET nucleation involved the use of aliphatic and aromatic sodium carboxylates to nucleate reinforced PET. Unlike the traditional inert solid physical nucleation agents such as talc which work by an epitaxial mechanism, these chemical nucleation agents work by dissolving in and reacting with PET. Extensive nucleation technology has been developed for PET involving the use of low molecular weight alkali metal carboxylates, as well as polymeric nucleants containing pendant alkali metal carboxylates (EP 185,981; EP 55,687; U.S. Pat. Nos. 4,349,503; 4,336,343; 4,753,975; EP 46,052; U.S. Pat. Nos. 4,483,955; 4,486,564; 4,351,757; EP 112,167).
U.S. Pat. No. 4,352,904 describes the combined use of a nucleant, i.e. "a sodium or potassium salt of a hydrocarbon acid containing between about 7 and 25 carbon atoms, or with the sodium or potassium salt of an organic polymer which contains pendant carboxyl groups . . . " and secondly, a mobility enhancer, a low molecular weight organic plasticizer (e.g. ketones, esters or sulfones). A filler is also added for the purpose of reinforcement of the resin. The presence of chopped glass fiber in injection moldable PET's markedly increases both the tensile strength and modulus of the PET, resulting in improved toughness.
The use of carbonyl containing plasticizers such as esters and ketones in reinforced PET in conjunction with selected sodium or potassium carboxylates enhances the rate of crystallization when the molded part has nearly cooled to the mold temperature. See U.S. Pat. Nos. 4,352,904 and 4,351,757. Additional mobility enhancers which have been developed include mixtures of oligoethers and oligoether segments employed jointly to synergistically increase mobility of the PET (U.S. Pat. No. 4,548,978). The addition of small amounts of polyolefins, for example, polyethylene, are also observed to enhance the mobility of PET in the melt, as well as improve its toughness (U.S. Pat. No. 4,303,573).
EP 21,648 describes the nucleation of PET by a variety of different basic salts of the alkali metals including carboxylic, phenolic, phosphonic, phosphinic, sulfonic, etc. This study describes the use of a range of ionizable metal salts which are able to activate the PET chain end and thus are useful in the nucleation of PET. The alkali metal salts function to nucleate PET. The mechanism by which alkali metal salts function to nucleate PET was best addressed by the spectroscopic study undertaken by Dekoninck et al (Polymer, 1989, 30:910). They were able to demonstrate by Fourier transform infrared spectroscopy (FTIR) that a basic sodium salt reacts with the ester to cleave the PET chain and to form a sodium carboxylate chain end. The sodium salt on the end of the PET chain then facilitates crystallization by precipitating from the melt, forming sites for nucleation. If the PET is annealed in the melt for extended periods of time, the salt gradually loses its nucleating ability, which occurs by the reaction of two neighboring sodium carboxylate ends to form disodium terephthalate, a very poor nucleating agent for PET.
Most group-1a carboxylates are effective as nucleants for PET along with many other group-1a metal salts. Two examples of group-1a carboxylates which were found to be totally ineffective as PET nucleants are disodium terephthalate and sodium parahydroxybenzoate. The precise reason that some salts are more effective than others is not understood, but is believed to depend at least in part on the ability of the salt to react with PET and transfer the metal to the chain end. This could involve such factors as solubility of the salt in molten PET, the heat of fusion of the salt crystal, and the relative basicity of the salt. Finally, chemical nucleation with ionic groups on chain ends is transferable to polymers other than PET. Legras et al., have shown how polycarbonate and polyetheretherketone (PEEK) can be nucleated by the presence of ionic groups on the chain end (Polymer, 1986, 27:109; Polymer, 1990, 31:1429).
Although progress has been made in finding nucleating agents for PET, a need continues to exist for improved nucleating agents for PET, in particular, agents which enhance the crystallization rate when cooling PET from a melt and also when heating PET from a glassy state.